I can't explain gravity but surely stacking up cable length plus true bypass pedals does not equal an increase in brightness?
as the pete cornish bit i posted up explains...
using say a 10 ft cable, into 10 true bypass pedals each connected by 1 ft of cable, followed by a 30 ft cable to the amp, means you have have (when all pedals are off) in total 50 ft of cable to drive using your guitar alone. cable capacitance is going to add up over 50ft, and that will definitely attenuate high end.
according to pete... this causes a loss of gain & treble which leads to the need to manually increase the gain and treble setting on the amp to compensate. which makes sense... because increasing cable length between the guitar and amp leads to a mellowing of the tone, a reduction in high frequency proportional to the length of the between guitar and amp. When you turn a pedal on, you add a buffer, which means your guitar is now driving the signal to that pedal, and that first pedal is buffering the signal to the amp... which in effect boosts the level and treble into the amp... leading to you to potentially hear more treble in the end, because you may have increased treble at the amp to compensate for the loss when not running any pedals (If all your pedals are true bypass pedals and switched off).
I'm using "you" figuratively.
The Pete Cornish thing makes sense. I believe its the cables characteristic impedance and capacitance over a given distance that works to roll off hi end. not increase it.
EDIT: also, I know you wouldn't join a pedal with 1ft of cable... it was just to make a point.